When purchasing foam, there are three major factors to consider:
SUPPORT: The foam should be able to support the weight put on it. One major factor in a foam's ability to support weight is the foam's thickness. Foam thinner than 3 inches will be a bit softer than stated and foam thicker than 6 inches will be a bit firmer than stated. The weight being placed on foam also affects its ability to support. Heavier people (270LB or more) will need a firmer foam to get proper support, and lighter people (75LB or less), can use a softer foam.
COMFORT: For most people, the softer a foam is, the better it feels and the more comfortable it is. However, foam that is too soft will not be as supportive and may lead to soreness, typically around the back. Ideally, you need a cushion that has a balance of comfort and support suited for both its use and your preference. Adding fiberfill wrap is an excellent way to increase softness and comfort without sacrificing supportive firmness.
DURABILITY: Durability or resiliency is the ability to maintain firmness and support throughout the life of a foam. Higher density foams have a much greater resistance to compressing, collapsing or bottoming out than lower density foams. High density foams are recommended when the cushion is used for heavy and/or everyday use. For applications with lighter and/or occasional use, lower density foam is usually recommended.